It is well-known that the liquid is polluted during the functioning of a power plant by the hydrogen which diffuses through the walls of the stream generators, as well as by the oxygen and hydrogen which are able, in particular, to originate from sodium/water reactions resulting from a leak in the steam generator. It is essential to remove these impurities in the sodium so as to limit corrosion of the structures by oxygen, the embrittlement of steel caused by hydrogen, but also to facilitate detection of the sodium/water reaction by maintaining a low hydrogen concentration, and so as to limit the risks of sealing with the hydride and oxide crystals which might form in the cold traps. To avoid this happening, a large number of models of cold traps have been considered, these traps generally comprising a tank where the liquid sodium is cooled below the crystallization temperature of the hydride and the oxide. These crystals become secured to the retention structures inside the trap, such structures being for example filters lined with metal wool.
The progressive fouling of the traps may mean that they need to be periodically regenerated by purging from them any solid impurities. The known method, which exists in several variants, has as one of its main characteristics a preliminary dumping of the liquid sodium which fills the cold trap and a heating of the emptied seat so as to thermally decompose the impurities. At 420.degree. C., the sodium hydride contained in the trap is decomposed according to the reaction (1): EQU NaH ---- Na+1/2.H.sub.2 ( 1)
The sodium freed by the reaction is in a liquid state and the hydrogen reacts with the sodium oxide according to the reaction (2): EQU Na.sub.2 O+1/2.H.sub.2 ---- Na+NaOH (2)
Caustic soda is thus formed which is then able to solidify (below 320.degree. C.) when heating ceases.
This method has several drawbacks. First of all, the reaction (2) is slow, it requires a prolonged treatment and, in practice, is impossible to carry out with the trap mounted on the reactor; therefore it would be proper to change the cold trap, if possible at the time of a programmed stoppage of the reactor, and to treat much later the trap full of impurities outside the reactor. Other difficulties arise from the presence of tritium hydrogen which is possible to reject into the atmosphere by only observing very strict rejecting norms, namely at extremely low flowrates; moreover, one part combines so as to form the soda whereas it may be more advantageous to isolate it so as to collect it. The evolution of gaseous hydrogen provokes an excess pressure (which moreover often needs to be maintained by a current of hydrogen in order to complete the chemical decomposition of the sodium oxide), which thus generates the need to continously control the pressure. With regard to the high temperature, the presence of soda results in a rapid generalized corrosion of the stainless steel wall of the trap and its lining. The gaseous hydrogen may also embrittle the steel. Finally, all the cold traps do not lend themselves to an easy emptying of the liquid sodium and the soda.
It is possible to avoid rejecting the tritium into the atmosphere by securing it to certain solid bodies. Unfortunately, the other drawbacks still remain.
However, the invention is seeking to regenerate the effectively and rapidly cold traps by totally eliminating impurities without having to unload the cold trap from the reactor. One essential object of the invention consists of separating the hydrogenated and oxygenic compounds so as to be able if need be to easily collect the tritium. The formation of soda is avoided, the corrolary of this being the absence of corrosion and which constitutes an essential difference from the European patent 0 012 074 of the same applicant, this patent describing a method to wash the trap with soda. This method facilitates dissolving the impurities in the soda, but the problems of corrosion have led this method to be abandoned.